Process,Structure And Performance Of Surface Nanocrystallization And Ion Nitriding On TC4 Titanium Alloy

With the development of China's marine power strategy,titanium alloy with excellent corrosion resistance has become a high-end material for marine engineering.However,titanium alloy itself has relatively low hardness,high friction coefficient,and poor wear resistance,which limit its wide application.In addition,there is a risk of crevice corrosion in the structural parts by titanium alloys in harsh marine environments.A multi-level ion nitriding layer on the surface of titanium alloy can simultaneously enhance its wear resistance and corrosion resistance.However,under extreme corrosion conditions(high temperature,low p H,high Cl~-concentration),the nitriding layer on titanium alloy may meet the problems of easy crack initiation and local spalling.In order to solve the above problems,this study plans to take surface nanocrystallization as a pretreatment before nitriding,to alleviate the structural change from the nitriding layer to the matrix through a gradient nanocrystalline structure,so as to suppress the peeling of the nitriding layer.Through the combination of surface nanocrystallization and ion nitriding on the surface of the titanium alloy,a corrosion-resistant nitriding layer with a good binding force is expected to better exert the function of the titanium alloy in the marine environment.In this study,the TC4(Ti-6Al-4V)titanium alloy with good comprehensive mechanical properties was used as the experimental material.The structure of the nitriding layer was adjusted by adjusting the process parameters of surface shot peening and low-temperature ion nitriding,to possess better film/base binding force and corrosion resistance.Through high-energy shot peening,gradient nanostructure was prepared on the surface of TC4 titanium alloy.The thickness of the deformed layer after shot peening(SPD layer and total deformation layer)increases stepwise with the increase of shot peening time or peening pressure,but the thickness of the deformed layer tends to be saturated after reaching a saturated deformation level.The average size of surface nanocrystals decreases with the increase of deformation,and tends to be stable when the deformation level is saturated.Grain refining mechanism is dominated by dislocation movement.Surface nanocrystallization significantly improves the surface hardness of TC4.When nanocrystallization of the surface?-phase enters the saturation stage,the surface hardness also tends to be stable.After shot peening at 0.6 MPa for 25 min,the size of surface nano-grains and the thickness of the SPD layer both begin to saturate.Therefore,SP 0.6 MPa/25 min is recommended as the process parameters of surface shot peening for TC4 titanium alloy to achieve surface nanocrystallization.Low temperature ion nitriding at 500?was achieved on surface nanocrystallized TC4titanium alloy,and a surface nanocrystalline nitriding layer was obtained.During the low temperature nitriding process at 500?,the stable nanocrystalline structure accelerates atomic diffusion,allowing nitrogen atoms to have a high diffusion rate at low temperature.The nitride layer thickness of the low-temperature nitrided SPed sample(SP-500?-20 h)is similar to that of the coarse-grained medium-temperature nitrided sample(O-600?-10h),which is about 2.1?m.At the same time the SP-500?-20 h sample and the O-600?-10 h sample have similar surface hardness,and the same hardness distribution at the cross-section.The SP-500?sample retains the surface nanocrystal structure,while the matrix grains also remains stable.Therefore,SP-500?low temperature nitriding is the recommended nitriding temperature in this study.If a thicker compound layer is required,the nitriding time can be extended.Scratch method was used to compare the film/base combination of the nanocrystalline nitriding layer(at SP-500?-20 h)and the coarse-grained nitriding layer(O-600?-10 h)with the same nitride layer thickness of about 2?m.The experimental results show that the critical stress Lc1 at which the surface of the nanocrystalline nitride begins to crack is small,while the nanocrystalline nitride layer and the coarse crystal nitride layer have the same critical stress Lc2 at which the layers fail completely,which is about 30 N.Considering that the surface roughness of the nanocrystalline nitride layer is much larger than that of the coarse-grained nitride layer,and the surface roughness adversely affect the scratch experiment,the film/base bonding force of the nanocrystalline nitride layer should actually be better.In 3.5%Na Cl solution at room temperature,the anode polarization current of nitriding samples with different surface states(SP-500?-20 h/O-600?-10 h)are lower than that of the non-nitriding samples,and the polarization resistance is higher,meaning a better corrosion resistance.The corrosion resistance of the nanocrystalline nitride layer(SP-500?-20 h)is significantly better than that of the coarse-crystalline nitride layer(O-600?-10 h).What is more,a higher self-corrosion potential and lower anode polarization current of the nanocrystalline nitride surface(N-SP)can be expected at the same sample roughness,when taking into account the negative effect of roughness on the polarization curve,that is,the nanocrystalline nitride layer can exhibit much better corrosion resistance than the coarse-grained one.